1. Field of the Invention
The invention relates to electro-optic display cells of the kind having an arrangement of display electrodes in contact with a liquid within a sealed enclosure and to methods of making such cells.
2. Description of Related Art
Electro-optic cells of the above kind generally employ either liquid crystal or electrochromic materials to produce a display effect. In both cases purity of the solution is vital to proper functioning of the display over its operational lifetime. Impurities can react chemically with components of the solution or interfere electrochemically with the operation of the cell.
Control of impurities requires not only that the initial components of the solution are pure but also that the cell receptacle be absolutely clean prior to assembly and hermetically sealed after being filled with solution. Processes of filling and of subsequently sealing the cell may expose the solution to contamination by, for example, sealing or atmospheric components. Another disadvantage of many sealing processes is that heat employed for curing, reflow of glass frit of soldering may also result in thermal decomposition of components or other heat damage. Exposure to the surrounding atmosphere during the filling process may also result in gas dissolving in the solution and later causing bubbles to form within the cell.
Very many alternative methods of filling and sealing display cells have been described in the prior art. By way of example, UK pat. No. 1,531,479 shows a filler hole in a steel backplate of an electrochromic cell. After filling, the hole is sealed by means of an overlying plate adhesively bonded to the backplate over and around the filler hole. U.S. Pat. No. 4,135,789 show a filler hole in the glass faceplate of a liquid crystal cell which is sealed by means of a capped glass plug fused in place with melted glass frit. U.S. Pat. No. 4,239,350 shows a combination of a glass plug and a plate bonded in place with epoxy resin. French published patent application No. 2,505,069 shows a combination of polythene and metal disks begin resin bonded over a filler hole in the glass cover plate of a liquid filled display.
Alternatively, cells can be filled via their side wall as in U.S. Pat. No. 4,199,228 which shows an electrochromic cell which is filled by means of a glass filler tube in a narrow glass side wall. The filler tube is sealed off by melting its outer end. U.S. Pat. No. 4,188,095 shows a display cell in which a filling port in a side wall formed of epoxy resin is subsequently sealed with solder.
Besides the sealing materials so far mentioned, it is also known to use resin material directly to seal a filling hole. In published patent application GB No. 2,046,935A the side wall of an electrochromic cell is formed by a double epoxy resin spacer and sealant the inner portion of which is cured by reaction with the electrolyte. The same resin sealant is used to seal a filling hole. In Japanese published patent application No. 55-138723A, an electrochromic cell is formed by a resin coated metal back plate and a glass cover. The plate is provided with a hole through which fluid is injected into the cell. The edges of the hole are also lined with resin and, after filling, the hole is sealed by melting the resin around it and applying further resin.
It is also known to provide a display cell with an elastomeric closure whose action is purely mechanical. U.S. Pat. No. 4,226,509 shows a liquid containing display of sandwich construction in which two filling holes are provided, one in each faceplate. A common elastomeric plug is drawn through both these holes to seal them. An alternative arrangement with a single filling hole is also described in which a sealing plug is drawn through a channel shaped brace on the exterior of the faceplate so as to cover the filling hole.
Another electrochromic display employing an elastomeric closure is described in U.S. Pat. No. 4,545,650. In this case an elastomeric diaphragm is placed over the filling hole when the cell is full and clamped in place in a manner which leaves it free to flex to accommodate expansion.